Ventilation systems in various industrial settings are usually required to have a certain minimum flow rates. For example, in commercial buildings a minimum level of air flow is required to maintain a healthy air quality within the building.
To ensure that the air flow requirements for a particular system are met it is desirable to be able to precisely measure the rate of air flow through the system. Systems without precise flow measuring capability are frequently overdesigned in terms of excess capacity. They are routinely operated at excess levels to ensure compliance with operating specifications. This adds unnecessary expense to the systems and their operation.
Further, system filters become clogged with particulates increasing pressure drop and robbing fan performance.
It is known that various methods of measuring air flow in a ventilation system can, in some cases, dramatically decrease the efficiency of the system. Traditional fan inlet devices (other than piezometer rings located outside of the air stream) can produce a dramatic fan performance drop from between 15% to 30%.
The efficiency of a ventilation system is a measure of how readily air flows through the system or, conversely, the system's resistance to air flow. Each component of a system through which air flows presents a certain amount of resistance to air flow. This resistance is determined by the size and shape of the component over which the air flows. Generally, components that are wider, smoother, straighter and shorter have less resistance to air flow, and therefore provide a more efficient system.
Improved efficiency can permit the use of a lower capacity fan to generate a given level of air flow in a system, and can require less energy to maintain a given level or air flow. In this manner, improving the efficiency of a ventilation system can reduce both the equipment and operating costs for the system. Many systems for measuring airflow, however, have just the opposite effect. Many flow measuring systems increase a system's resistance to air flow, and thereby reduce the system's efficiency.
Existing air flow measurement systems have had to balance the trade-offs between efficiency and precision of measurement. Existing measurement systems typically create an obstruction or constriction within the air flow, and measure the effect of the obstruction or constriction on the air pressure at a certain point in the system. Increasing the size of the obstruction or amount of the constriction generally increases the precision of the flow measurements but also increases the negative impact of the measuring system on the system efficiency.
For example, a pitot tube measuring system is an obstruction-type measurement device. A typical pitot tube has an orifice facing directly upstream to provide a total pressure measurement and an orifice oriented to provide a static pressure measurement. From this information, the velocity of the air stream can be determined. However, each pitot tube creates a disturbance in the air flow, thereby increasing turbulence and resistance and decreasing efficiency.
A venturi tube measuring system is another example of an air flow measurement system that operates by constricting the air flow in the system. A typical venturi tube has an inlet diameter which narrows down to a throat of a smaller diameter. The smaller cross-sectional area at the throat results in an increase in air velocity. A pressure tap monitors the pressure at the inlet, and a second pressure tap monitors the pressure within the throat. This pressure differential is then used to estimate the flow rate.
A measuring system similar to the venturi tube may have a limited aperture for constricting the air flow within a conduit. A pressure sensor is generally located upstream from the aperture and another pressure sensor is located downstream from the aperture. The pressure differential can be used to determine the approximate air flow through the aperture. This type of flow sensor typically creates a significant pressure drop in the air stream that can dramatically reduce system efficiency.
Recently, technological advances in the thermistor industry have made it possible to use ceramic reinforced glass body thermistors for stable and repeatable use in thermal dispersion type air measurement applications.
Representative of the art is U.S. Pat. No. 5,586,861 which discloses a centrifugal fan provided with an inlet cone that serves to measure air flow through the fan. The inlet cone has a flared inlet for receiving air, a narrow throat, and a flared outlet for expelling air into the center of a rotating fan wheel. Pressure taps are provided to measure the static pressure at the inlet and the throat. The difference between these pressures, adjusted for the empirically determined characteristics of the inlet cone, can be used as an indication of air flow. In one aspect of the invention, a controller monitors the pressure differential, calculates a flow rate based on the characteristics of the cone, and adjusts the fan speed to maintain a desired air flow.
Reference is made to applicants pending U.S. application Ser. No. 12/286,930 filed Oct. 3, 2008 for a Gas Measurement System.
What is needed is a fan air flow measurement system comprising a thermistor pair mounted in sensor housing which is flush mounted in a centrifugal fan inlet bell. The present invention meets this need.